I have to confess, I thought this would not have any impact. But I did try it. I honestly don't know if the particular RCA-XLR cables I ordered are wired correctly (I got WBC/"World's Best Cables" off Amazon because it would arrive quickly and I could therefore test it quickly).
The cables have not totally eliminated the hum - but they have definitely had a positive effect - thanks! After connecting them, I ran my previous test: all components connected to each other, turn on the power amp with the EQ plugged in to AC but turned off and the Oppo unplugged from AC. No hum, just as before. Then I plugged in the Oppo but did not turn it on. Previously this immediately would produce hum in the speakers. Now the speakers remain totally silent when I plug in the Oppo - progress!
When I turn on the EQ, I hear a little bit of the EQ's self-noise in the form of low-level hiss if I put my ear about 6-12 inches away from the speaker (I posted about this in a thread about Hypex and Purifi amps - my Purifi amp is so quiet that I now hear the upstream components' self-noise added to the signal when I power them on). But still no hum.
When I turn on the Oppo and it fully boots up, I do unfortunately hear hum in the speakers. However, it is significantly reduced from before. If I turn my head sideways to the speaker, so that my ear opening is directly facing the speaker's drivers, I can hear the hum from 6-8 inches away for the L speaker and about 12 inches away for the R speaker. (I am guessing this small difference reflects a tiny difference in the amplitude of the hum, perhaps related to internal wiring arrangement in one or more of the components). If I turn my head to normal position so I'm looking at the speaker, or if I back away from that 6-12 inch range, I lose track of the hum and can't hear it anymore. And so from my listening position, which is about 8-9 feet/2.5-3M away, I can't hear anything, which of course is what is important.
I am still testing to see if the hum increases when I've been playing music for awhile (maybe temp related somehow?). And I have not yet had a chance to listen at night (when human hearing gets more acute), but so far this is encouraging.
Most importantly, the Hum-X is still out for delivery to me today, so I haven't yet had a chance to try that. Since these cables have had an effect, though, it increases my cautious optimism (however rationally or not) that the Hum-X also will have a positive impact.
Two final questions, for you,
@DonH56 , and anyone else who has a sense of this:
1. Would it have any potential additional benefit to also install RCA-XLR cables between the Oppo and the EQ? I would guess No, since the XLR-RCA connection would be going in the opposite direction as the EQ-amp connection and therefore would not have the same benefit. But as noted previously I'm kind of an idiot when it comes to grounding and similar electrical basics so I really have no idea.
2. Similarly, my plan is to connect the Oppo to the Hum-X since it's the obvious choice - it's the only low-current, 3-prong component in the chain. But would it be worth trying the Hum-X (or a 2nd Hum-X) with the EQ too? It's 2-prong, floating, but since I don't know exactly how the Hum-X works, I wonder if there's any possible benefit since the ground hum is traveling through the EQ to get to the amp and speakers. Again, I doubt this would be of any use but I really don't have any expertise in this area, so...
I'm glad I feel I can once again listen to my system without feeling like I'm hearing a tiny bit of that hum during the silence between tracks, but I would definitely like to basically eliminate it altogether if I can. And this is definitely getting me into gear to prep my spare Oppo 205 for sale to make a dent in my Genelec Endgame Savings Fund.
Thanks!